Action potential movement through an axon A cross-section of an axon, with an action potential (AP) moving from left to right. The AP has not yet reached point 4; the membrane there is still at rest. At point 3, positive sodium ions are moving in from the adjacent region, depolarizing the region; the sodium channels are about to open. Point 2 is at the peak of the AP; the sodium channels are open and ions are flowing into the axon. The AP has passed by point 1; the sodium channels are inactivated, and the membrane is hyperpolarized.

Active site The active site of the penicillin-binding protein. The gray stick-like structures represent the secondary and tertiary structure of the penicillin-binding protein. Binding of the antibiotic, the substrate, to the active site blocks the normal action of the protein in the bacterial cell, resulting in death of the cell.

Alternative splicing More than one protein can be made from a gene. In this case, three different mRNA molecules are made from one gene. The exons (the numbered boxes) can combine in different configurations to eventually form different proteins.

Artiodactyl An artist's rendering of an ancient artiodactyl, a group of split-hoofed animals that were the ancestors to camels, hippos, pigs, and probably modern whales.

Ascent of man An early view of the evolution of man from its ancestors

Asteroid A re-creation by NASA scientists of the impact made by an asteroid 500 miles in diameter at Chicxulub, on the Yucatan Peninsula. This impact is thought to be the cause of the K/T mass extinction 65 million years ago.

Attached bacteria Scanning electron micrograph of a young bacterial biofilm that formed on the lower surface of an oil slick on the Athabasca River.

Attached bacteria Scanning electron micrograph of a young bacterial biofilm that formed on the lower surface of an oil slick on the Athabasca River.

Bacterial conjugation The F-pilus serves as a point of contact between a bacterium containing an F-plasmid (the "male") and a bacterium lacking the plasmid (the "female"). After the female cell is contacted the pilus retracts, pulling the cells together. The exact mechanism of DNA transfer from male to female is not known; it may be by a channel in the pilus or by a temporary fusion of the mating cells.

Bacterial transformation 1. Naked DNA fragments from disintegrated cells in the area of a potential recipient cell. This cell must be of the correct genus and be in a state of competence, allowing the entry of the DNA fragments. 2. Entry of naked DNA into competent cell. 3. Recombination.

Carl Woese's data An autoradiogram of a gel electrophoresis separation and visualization of ribosomal RNA, part of Carl Woese's research that led him to reorganize the Tree of Life into three domains rather than five kingdoms.

Chloroplasts A photomicrograph of chloroplasts, key cellular structures found in plants, which originally existed as free-living bacteria

Cloning animals by nuclear transfer A donor cell is fused with enucleated egg cell by subjecting the two cells to pulses of electricity. The cell replicates in culture, generating an embryo, which is then introduced into the uterus of a female for development.

CLUSTAL data A CLUSTAL alignment of a segment of a gene from four species. The red letters show the amino acid sequence (R=arginine, P=proline, G=glycine, etc.). The nucleotides that are conserved in all four species are shown in the columns with an asterisk at the bottom.

Colinearity Genes that are expressed at the anterior end of an animal are located at the more anterior region of the chromosome. Likewise, posteriorly expressed genes reside on the posterior end of the chromosome. This is referred to as spatial colinearity.

Confocal neuron A reconstruction of a neuron in a brain slice using confocal microscopy

Coordinate, gap, pair-rule, and segment polarity genes The cascade of developmental genes in segmentation in Drosophila. Maternal effect/coordinate genes set the anterior-posterior axes. The embryo is subdivided into progressively smaller regions by the actions of each class of segmentation genes.

Corn lilly A substance called cyclopamine, found in the corn lilly plant, Californicum veratrum, is the cause of birth defects in lambs.

Cyclopic lamb This lamb's mother ate a poisonous plant, the corn lilly, which contains a compound called cyclopamine. This compound inhibits the action of the Sonic Hedgehog protein, which is involved in the formation of the neural system.

David Reimer and his brother This is the first family gathering after David Reimer announced his intention to live as a male instead of a female. (David is on the left.)

Dopamine transporter Left: Dopamine in the synaptic space binds to dopamine receptors on the post-synaptic cell. Dopamine transporters in the presynaptic membrane take up the dopamine molecules from the synaptic cleft and return them to the presynaptic cell. Right: Cocaine blocks the reuptake of dopamine, leading to molecular changes that contribute to addiction.

Drosophila embryo with bicoid protein stain This is a two-hour-old Drosophila embryo that shows the expression of the bicoid protein. The bicoid protein forms a gradient with the highest expression at the anterior end (left side in this photo) of the embryo.

Drug binding to active site of protein In virtual ligand screening, the three-dimensional image of the protein is fed into a computer, which attempts to fit millions of small molecules to a targeted active site. Small molecules that bind well to the protein become good leads for potential new drugs.

Fruit fly with extra eyes The head of a fruit fly, Drosophila melanogaster, viewed by scanning electron microscope (380x magnification). Targeted expression of the eyeless gene induced the formation of the eye facets on the antenna (to the lower-right of the eye), which are very similar to the facets of the normal eye. This identifies eyeless as the master control gene of eye morphogenesis.

Gene disruption by homologous recombination The plasmid contains a gene interrupted by a marker gene (XR). Recombination involving two crossovers between the plasmid and wild type chromosomal DNA with the interrupted gene and the selectable marker.

Gingerich's revised picture of whale evolution In the revised picture of whale evolution, cetaceans evolved from the artiodactyls, and not the mesonychids, as previously thought. The closest living relative to whales is now thought to be the hippo.

Group A streptococci Electron micrograph of an ultra-thin section of a chain of group A streptococci. The cell surface fibrils, consisting primarily of M protein, are clearly evident. The bacterial cell wall, to which the fibrils are attached, is also clearly seen as the light-staining region between the fibrils and the dark-staining cell interior. Incipient cell division is also indicated by the nascent septum formation (seen as an indentation of the cell wall) near the cell equator. The streptococcal cell diameter is equal to approximately one micron. (20,000x magnification)

Michael Bishop and Harold Varmus Michael Bishop, PhD, and Harold Varmus, PhD. They received the Nobel Prize in 1989 for their discovery that normal cells contain genes capable of becoming cancer genes.

Head of Drosophila melanogasterDrosophila, the fruit fly, is a common model organism used in genetic research. It was one of the organisms sequenced in the Human Genome Project.

Heap leach pad Heap leach operations, like this one operated by Kennecott, can contain a million tons of ore, piled a leak-proof pad. Adding water on top promotes the growth of microbes and starts the bioleach.

Helper T cells regulate both humoral and cellular immunity A specialized macrophage ingests foreign antigens and displays antigen fragments along with MHC (self) molecules on its surface. A helper T cell (TH) with the appropriate receptor binds and responds by producing cytokines that stimulate antigen-specific B cells, as well as specific cytotoxic T cells.

Hemagglutinin protein-pH 5 When the influenza virus enters a cell it is engulfed into an endosome. The pH inside the endosome lowers from 7 to 5, causing the hemagglutinin protein to change shape, open the endosome membrane, and let the viral material into the cell.

Hemagglutinin protein-pH 7 When the influenza virus attaches and enters a cell, the hemagglutinin surface protein is at pH 7. A shape change caused by a lower pH is responsible for the hemmagglutinin to open the endosome and release viral material into the cell.

Hippocampal neurons Two hippocampal neurons labeled with green fluorescent protein, viewed with confocal microscopy. Such neurons release and sense glutamate and engage in long-term potentiation (LTP). Note the synaptic connections between the lateral processes of the two neurons.

HIV viruses HIV-1 by transmission electron microscopy, showing the mature virus particles that have budded off the nearby lymphocyte

Human and chimp allele clustering For nearly all genes, human alleles cluster together and chimp alleles cluster together (left). In the case of the major histocompatibility complex (MHC), human alleles are often more closely related to chimp alleles and vice-versa. This occurs due to balancing selection maintaining variation at the MHC (right).

Hypotheses of human migration Left: The "Out of Africa," or "Replacement," hypothesis suggests all living humans evolved from a group that originated in Africa. Right: The "Multiregional" hypothesis suggests several groups evolved in parallel to form today's population of humans.

Icelandic family genealogy text Icelandic people have kept careful records of their family histories for hundreds of years, aiding efforts to track disease patterns in their genes.

Janet Reimer holding her twins, Bruce and Brian Brenda and Brian are identical twins. Brenda was born a male and named Bruce. After a botched circumcision, scientists believed it was best to change him into a girl. This case, termed the "Joan/John case" in medical literature, has been used often in the argument for nurture's role in the gender identity. At 14, Brenda shed her upbringing as a female and began to live as a male. He took the name David, and has said that he never felt right as a female, disproving what the scientists believed: that nurture could fully override his genetic makeup.

John Kendrew John Kendrew determined the structure of the myoglobin protein in 1957. He shared the 1962 Nobel Prize in Chemistry with Max Perutz, who determined the structure of hemoglobin.

Leroy Hood Leroy Hood, MD, PhD, developed the automated genetic sequencer in 1986 and was a strong proponent of the Human Genome Project. He is the founder and president of the Institute for Systems Biology in Seattle, Washington.

Max Perutz Max Perutz determined the structure of the hemoglobin protein in 1959. The work took him 22 years. He shared the 1962 Nobel Prize in Chemistry with John Kendrew, who discovered the structure of myoglobin.

Microarray experiment A. RNA is isolated from cells from two samples (in this illustration, infected and uninfected plant cells). B. The mRNA from both samples is copied to a more stable form, called cDNA, using reverse transcriptase. C. At the same time, the cDNA is labeled with fluorescent tags (a different color tag for each sample). D. The tagged cDNA is placed on the microarray chip, where it binds to the corresponding DNA that makes up the genes that have been previously spotted on the chip. E. The chip is placed in a laser scanner, which identifies the genes that hybridize to each sample (uninfected=green; infected=red; and both samples=yellow). F. The data are displayed on a computer screen where expression of the individual genes can be identified.

Monophyly and diphyly of bat evolution Alternative possibilities of bat phylogeny. Top: Bats form a monophyletic clade, in which flight evolved once in mamals. Alternately, bottom, bats are diphyletic, and flight evolved twice in mammals.

Open reading frames To find an open reading frame (ORF), a computer program identifies start codons (red arrows) and stop codons (green lines) in all three reading frames (represented by the three stacked rows). The black box is the largest ORF found in this sequence.

Pachyaena An artist's rendering of Pachyaena, an example of the extinct group of animals called mesonychids. Once thought to be the ancestors of modern whales, it now appears that this is not the case.

Pancreatic islets The endocrine portion of the pancreas takes the form of many small clusters of cells, called islets of Langerhans or islets. Humans have roughly one million islets. In standard histological sections of the pancreas, islets are seen as relatively pale-staining groups of cells embedded in a sea of darker-staining exocrine tissue. This image shows three islets in the pancreas of a horse.

Parsimony Three possible unrooted trees are shown. The top tree assumes nine changes in character state occurred (each change is represented by a mark), the middle tree assumes ten changes, and the bottom tree assumes eleven. Because the top tree assumes the fewest changes, it is the most parsimonious tree.

Passenger pigeon Once a common bird of eastern North America, the last passenger pigeon died in a zoo in 1914.

Paternal and maternal lineages Maternal lineages can be traced through mitochondrial genes, which are inherited by males and females only from the mother. Paternal lineages can be traced through the Y chromosome, which is inherited only by males and only from the father. (M=male and F=female)

Pathways leading to sex development in mammals The genital ridge in an embryo is converted to a bipotential gonad by the products of the LHX9, SF1, and WT1 genes. This gonad develops into an ovary under the influence of the WNT4 and DAX1 gene products; it develops into a testis under the influence of the products of the SRY and SOX9 genes. The ovary produces cells that make estrogen, which causes the Müllerian duct to differentiate into the female genitalia. The testis makes two hormones, anti-Müllerian duct factor (AMH), which causes the Müllerian duct to regress, and testosterone, which causes the Wolffian duct to differentiate into male internal organs. Testosterone is also converted into dihydrotestosterone (DHT), which is required for development of male external genitalia.

Phagocytes on a biofilm Scanning electron micrograph of the outer surface of a Tenckhoff catheter recovered from the peritoneum of a chronic ambulatory peritoneal dialysis (CAPD) patient. It shows the extensive microbial biofilm that had developed and a large number of phagocytes that were attracted to this surface by the presence of sessile bacteria.

Plasmid DNA A photomicrograph of plasmid DNA, a small circular form of DNA found in bacteria

Polymerase chain reaction (PCR) 1) Double-stranded DNA in the sample is heated to generate single strands. 2) Sequence specific primers are added, which anneal to desired sites on the DNA. 3) Nucleotides and heat-tolerant DNA polymerase allow for primer extension at elevated temperature. 4) The result is two new copies of double-stranded DNA. The process is repeated to generate multiple specific dsDNA molecules.

Protein structure Primary Structure: The specific sequence of amino acids in a polypeptide chain. Secondary Structure: The folding of the polypeptide chain into specific shapes, such as the alpha helix and beta pleated sheet. Other regions of secondary structure may include turns and random coils. Tertiary Structure: The unique three-dimensional shape that is the result of chemical interactions between amino acids that fold the regions of secondary structure. Quaternary Structure: The specific interaction of two or more polypeptide subunits.

Pseudomonas aeruginosa Colonization of P. aeruginosa on a human lung. This bacterium can cause chronic lung infections in people with cystic fibrosis.

Ras protein The model structure of the proto-oncogene ras. Ras plays an important role in the signal transduction pathway that is involved with cell growth. Mutations in Ras have been implicated in many cancers.

RNA processing A gene consists of coding regions, called exons, that are interrupted with intervening noncoding regions, called introns. During transcription, the whole segment of DNA that corresponds to a gene is copied to make RNA. During RNA processing, the introns are removed and the exons are joined. A poly(A) tail is added to the mRNA.

Selective breeding of maize The ancestor of modern corn had tiny kernels, each protected by a tough husk. Domestication of maize, which began thousands of years ago, selected for large sheathed cobs containing large kernels without husks.

Signal transduction pathway A signal (in this example, a growth factor) binds to a tyrosine kinase receptor on the outside of the cell. This activates the membrane protein (through the addition of phosphate groups), which in turn activates proteins in the cytoplasm such as kinases. Several other proteins may be involved in the cascade, ultimately activating one or more transcription factors. The activated transcription factors enter the nucleus, where they stimulate the expression of the genes that are under the control of that factor. This is an example of the RAS pathway, which results in cell division.

Southern blotting 1) DNA fragments are generated using restriction enzymes. 2) The fragments are separated in a gel by the application of an electric charge. 3) The fragments are then blotted onto a piece of nitrocellulose paper, where they retain their same pattern of separation, but are denatured to become single-stranded DNA. 4) A unique single-stranded portion of the gene of interest (the probe) is radioactively labeled and allowed to anneal with the blotted paper. 5) When exposed to a sheet of photographic film, any DNA fragments that annealed with the labeled probe are identified.

Synapse Synaptic vesicles fuse with the presynaptic membrane to release neurotransmitter into the synaptic space. Here, they bind with neurotransmitter receptors in the postsynaptic membrane.

T. rex Illustration of Tyrannosaurus rex, a casualty of the mass extinction at the end of the Cretaceous period

Telomeres Human chromosomes with the telomere tips stained. The telomeres are protective ends of the chromosomes that get shorter each time a chromosome is replicated.

The cell cycle The cell cycle is an ordered process of events that occurs in four stages. During the two gap phases, G1 and G2, the cell is actively metabolizing but not dividing. In S (synthesis) phase, the chromosomes duplicate as a result of DNA replication. During the M (mitosis) phase the chromosomes separate in the nucleus and the division of the cytoplasm (cytokinesis) occurs. There are checkpoints in the cycle at the end of G1 and G2 that can prevent the cell form entering the S or M phases of the cycle. Cells that are not in the process of dividing are in the G0 stage, which includes most adult cells.

The evolution of the Y chromosome The degeneration of the Y occurred in four discrete episodes, beginning about 300 million years ago when a reptile-like ancestor acquired the SRY gene on one of its autosomal chromosomes. Each of the four episodes involved a failure of recombination to occur between the X and the Y chromosomes, resulting in subsequent decay of some genes in the non-recombining region.

The life cycle of the malaria parasite Sporozoites are delivered to the human bloodstream from the salivary gland of the Anopheles mosquito when the insect bites. In the liver, the sporozoites multiply and become merozoites. The merozoites enter red blood cells and become trophozoites. Red blood cells rupture and new merozoites, which have developed from the trophozoites, are released. Gametocytes (the sexual stage) are eventually produced. Gametocytes taken up by the mosquito in a blood meal fuse to form zygotes, which give rise to sporozoites.

The methane cycle Methanogens are intolerant to oxygen so they thrive in anoxic sediments. The methane they produce is a carbon and an energy source for methane oxidizers in overlaying water.

The nitrogen cycle Bacteria are key to the cycling of nitrogen in ecosystems. Different species are involved in decomposition and ammonification, nitrification, denitrification, and nitrogen fixation.

The replication cycle of HIV 1) Membranes of the virus and the host cell fuse, and viral RNA and reverse transcriptase enter the host¡s cytoplasm. 2) Reverse transcriptase allows viral RNA to be copied to DNA. 3) Viral DNA is incorporated into the host chromosome as provirus. 4) Transcription and translation of viral proteins: viral RNA becomes incorporated into viral particles and is transcribed as well. 5) Viral particles bud out of the host cell, acquiring an envelope in the process.

The "Shrub of Life" Proposed by W. Ford Doolittle, this view of early evolution suggests multiple primitive cells as ancestors to the three domains, and illustrates lateral gene transfer among early organisms.

The structure of HIV Shows the binding of HIV to a host cell. gp120 on the virus binds CD4 receptors on the host. A second coreceptor molecule on the host is also required for binding.

Transposons Transposons facilitate the movement of genes throughout bacterial populations. These jumping genes can move from a chromosomal location to a plasmid. Conjugation allows the resultant plasmid to move readily within the bacterial population.

Trilobite Trilobite, a casualty of the mass extinction at the end of the Permian period.

2D gel of proteinsHaemophilus influenzae cell proteins separated by 2D gel electrophoresis. The basic proteins are to the right of the gel and the acidic proteins to the left. High molecular weight proteins are to the top of the gel.

Undifferentiated embryonic stem cells Microscopic (5x magnification) view of a colony of undifferentiated human embryonic stems cells. The embryonic stem cell colonies are the rounded, dense masses of cells. The flat, elongated cells in between the embryonic stem cell colonies are fibroblasts that are used as a "feeder layer" on which the embryonic stem cells are grown.

Whorls The tissues that will become floral organs are arranged in concentric whorls of a developing flower.

X and Y chromosomes The Y chromosome is the smallest chromosome in humans. While most chromosomes have about 1,500 genes, the Y has less than 200.

Y chromosome The Y chromosome is very small compared to the X chromosome. The pseudoautosomal regions at the tips contain the genetic material on the Y that shows similarity to the X chromosome. The SRY gene is located on the p arm of the Y.

Zebrafish embryo This 18-hour-old zebrafish embryo is labeled with a probe that shows the expression of sonic hedgehog mRNA.